In the field of refrigeration and air conditioning, 1,1,1,2-tetrafluoroethane (R134a) that is a hydrofluorocarbon (HFC) and R410A that is a mixed refrigerant of difluoromethane (R32) and pentafluoroethane (R125) in a mass ratio of 1/1 are currently widely used as refrigerants for a refrigerator, a vehicle air conditioner, a room air conditioner, industrial refrigerating machine, and the like. Although such HFC refrigerants have an ozone depletion potential (ODP) of zero, they have a high global warming potential (GWP) of 1000 or more, and therefore, their usage is limited in accordance with what is called the F-gas regulations aiming at the global environmental protection.
As a substitute for a refrigerant having a high GWP, single use of difluoromethane (R32) is under examination as a candidate owing to its thermodynamic characteristic. Although the GWP of R32 is 675 and rather high, it is under examination as a potential candidate because its gas pressure is high and it is a highly efficient refrigerant.
In using a refrigerant increasing a pressure, such as R32 or a mixed refrigerant containing R32, since the discharge temperature in a compressor becomes high, an oil film of a refrigerating machine oil for attaining lubrication in the compressor becomes thin to cause a severe lubrication condition, and therefore, a refrigerating machine oil having good lubricity is necessary.
Patent Literature 1 proposes, as a refrigerating machine oil for an R32 refrigerant, an ester synthesized from trimethylolpropane and/or neopentyl glycol, a specific dibasic acid, and a monohydric alcohol or a monocarboxylic acid, and Patent Literature 2 proposes, as one for an R32 single refrigerant or a mixed refrigerant containing 50% or more of R32, an ester-based refrigerating machine oil that contains 20 to 60% by mass of a monobasic fatty acid having 5 or less carbon atoms, uses a branched fatty acid as a fatty acid having 8 or more carbon atoms, and has a viscosity at 40° C. of 32 to 100 cst. With respect to a base oil for a general industrial lubricating oil, Patent Literature 3 proposes a lubricating oil base oil containing a synthetic ester obtained by reacting an alcohol component containing 90% by mass or more of trimethylolpropane with a carboxylic acid component that contains a monocarboxylic acid having 8 to 12 carbon atoms and adipic acid, and further contains, in a total amount of 90% by mass or more, caprylic acid and/or a monocarboxylic acid having 8 to 12 carbon atoms containing 90% by mass or more of caprylic acid in total, and adipic acid.